Internal combustion engines used in motor vehicles are continually being optimized with respect to exhaust emissions and fuel consumption. One source of avoidable fuel consumption as well as avoidable exhaust gas and noise emissions is urban driving. In urban driving, due to the frequent stopping at traffic lights and intersections, internal combustion engines in motor vehicles spend a not inconsiderable part of their operating time idling. This represents a considerable waste of resources and an environmental burden, because it results in an unnecessary overconsumption of fuel and therefore in an associated emission of poisonous, climate-relevant, and injurious exhaust gases. To rectify these unnecessary idling phases, motor vehicles are equipped with start/stop automatic systems, the internal combustion engine being shut down in the presence of specific stop conditions and being started up once again electrically in the presence of specific start conditions. Using start/stop systems of this type, it is possible to significantly reduce the idling phases of internal combustion engines in urban driving.
Start/stop systems for automatically shutting down internal combustion engines have been made known from European Published Patent Application No. 0 233 738, German Patent No. 33 43 018, and German Patent No. 44 21 512. Although drive systems having start/stop automatic systems have proven their basic suitability for everyday use, they are nevertheless affected by disadvantages. Among the latter are a loud starter noise, which derives essentially from the transmission noises of the pinion gear transmission of the high-gear-ratio starter motor (pinion gear-toothed gear ratio 1:9 through 1:15, auxiliary transmission gear ratio 1:3 through 1:6, overall gear ration 1:30 through 1:60). Due to actuating the starter for the internal combustion engine, which occurs frequently in urban driving, an increased noise disturbance arises for the vehicle occupants as well as for the residents of areas in inner cities near traffic lights and intersections. In addition, components such as electromagnetic switches, pinion gears, meshing devices, as well as electromotors experience enormous wear in response to frequent actuation in urban driving. To limit the wear, in known drive systems, overdimensioned starters are used, which require a great deal of installation space and which, because of their own great weight, negate a portion of the fuel economies achieved by shutting down the internal combustion engine.
French Published Patent Application No. 2 563 280 discloses, in a drive system for a motor vehicle, not to shut down the vehicle internal combustion engine in drive pauses, but rather to continue to have it rotate using an electrical machine, so as to provide auxiliary aggregates with mechanical drive energy. The electrical machine in accordance with this solution also functions as a starter for the internal combustion engine.
U.S. Pat. No. 4,797,602 discloses a starter/generator which operates in conjunction with a start-stop control system. The starter/generator has a gear ratio with respect to the internal combustion engine of 4:1. The starter/generator runs on a voltage that is increased in comparison to an automobile electrical system, an energy storage unit being provided at the increased voltage level.
European Patent No. 0 847 494 relates to a drive system, in particular, for a motor vehicle, as well as to an operating method. The proposed drive system includes an internal combustion engine as the drive aggregate, and also an electrical polyphase machine, that is directly coupled to a drive shaft of the drive aggregate, or is capable of being so coupled. The polyphase machine rotates at the identical speed of the drive shaft, and it is configured so that it can start as a drive aggregate, starting from a standing position. At least one inverter is provided, which generates for the magnetic fields of the electrical machine voltages and/or currents of variable frequency, amplitude, and/or phase. The inverter has an intermediate circuit having a voltage level that is elevated with respect to the electrical system of the motor vehicle, provision being made in this electrical system for an energy storage device for storing electrical energy. In addition, a further energy storage device is provided, from which energy is taken for starting, and which is an electrical energy storage device at the elevated voltage level of the intermediate circuit and/or an electrically operated flywheel storage unit electrically connected in parallel via the intermediate circuit to an intermediate-circuit capacitor at the elevated voltage level. In addition an automatic start/stop control unit is provided, in the context of which the drive aggregate is started using the electrical machine.
The advantage to be achieved through the method proposed in accordance with the present invention is to be seen above all in the fact that among the boundary conditions for achieving a reliable starting function in a cold start and for achieving a rapid, as far as possible noiseless warm start, the electrical power to be generated by the vehicle battery can be reduced up to 20%. The electrical machine used as the starter/generator, which can be operated on an inverter or on an indirect a.c. converter, is limited before reaching an imaginary turning point, i.e., before the intersection of the characteristic curve for the mechanical power of the starter-generator and the starting torque characteristic curve, by limiting the phase current of the electrical machine. As a result of the limiting, it is achieved that the electrical power that can be made available by the vehicle battery does not at any time point exceed electrical power Pel required at the stationary operating point. In this way, it is achieved that the energy storage unit can be optimally adjusted to electrical power Pel of the starter-generator in the latter""s stationary operating point.
The prompt limiting of the electrical machine at a limiting point and operating it at an electrical limiting power curve determined as a function of the status parameters of a motor vehicle battery prevents an electrical machine functioning as the starter-generator from operating at an imaginary operating point, at which a far greater electrical power Pel is necessary, in comparison with electrical power Pel actually required in the stationary operating point of the electrical machine. Abandoning the torque curve of the starter-generator and operating the latter along the electrical limiting power curve yield a reduction in the starting torque in a narrow rotational speed range of the internal combustion engine. Above a rotational speed of roughly 50 minxe2x88x921, the starting torque is below the value at the stationary operating point and always above the torque requirement, so that the starting function of the internal combustion engine is assured even in cold-start conditions.
One improvement of the cold start function, or a power reduction, can be achieved in coordination with the thermal design of the inverter of the electrical machine, by setting an elevated torque below the electrical limiting power curve, at small rotational speeds.
The control/regulation of the power limiting of the electrical machine is accomplished as a result of the fact that the maximum electrical battery power is taken from a measurement of the battery terminal voltage in conjunction with a limiting value regulation of the phase current at the voltage lower limit. The obtained maximum electrical battery power determines the electrical limiting power curve and the position of the limiting point on the torque curve of the starter-generator.
If a claw pole generator is used as the starter-generator, then the phase current on it can be reduced when a predetermined rotational speed is reached. A different possibility for the power limitation of a claw pole generator having an indirect a.c. converter lies in influencing the angle (load angle) between the phase voltage and the phase current space vectors when a predetermined rotational speed of the internal combustion engine is reached.